A downhole motor comprising a stator with a rotor rotatably mounted therein is often used during well bore operations to drive a rotating tool, such as, for example, a cutting tool designed to mill through casing and/or drill into a formation. Such downhole motors may be run into the well bore as part of a bottomhole assembly comprising other components positioned above and below the motor. Therefore, unless the rotor is “locked” to prevent it from rotating with respect to the stator, relative rotation of the bottomhole assembly components above and below the motor is possible. Such relative rotation is undesirable, for example, when bottomhole assembly components disposed below the motor must be properly oriented and anchored into place before milling or drilling commences.
For example, the downhole motor may be part of a sidetracking bottomhole assembly designed to drill a deviated, and sometimes horizontal, lateral well bore from a main well bore. One such sidetracking bottomhole assembly is shown and described in U.S. application Ser. No. 09/303,049, filed Apr. 30, 1999 and entitled “One-Trip Milling System”, hereby incorporated herein by reference. A sidetracking bottomhole assembly may comprise, for example, a drilling assembly that includes a cutting tool, a whipstock, and a hydraulically settable anchor or packer all disposed below the motor. In operation, the sidetracking bottomhole assembly is lowered into the well bore, the whipstock is angularly oriented, and the anchor or packer is set. Then the cutting tool is rotated by the motor and guided along the whipstock while the drilling assembly moves downwardly within the main well bore. The whipstock has a ramped or sloped surface whereupon the cutting tool is deflected in the direction of the lateral well bore as the cutting tool moves downwardly. In a cased well bore, for example, the whipstock ramp urges the cutting tool radially outwardly so that the cutting surfaces of the tool engage the casing and mill a longitudinal window therethrough. The whipstock ramp further urges the cutting tool radially outwardly such that the cutting tool is positioned entirely outside of the well bore casing, thereby completing the window. The lateral well bore is drilled through this window, out into the formation.
Accordingly, to cut the casing window in the correct location, it is critical to properly angularly orient the whipstock in the main well bore so that the whipstock ramp faces in the desired direction. Then the hydraulically settable anchor or packer must be set to maintain the desired orientation of the whipstock. To hydraulically set the anchor or packer, drilling fluid is displaced through the bottomhole assembly, including the motor. This fluid displacement is likely to drive the motor during the setting procedure, which would thereby rotate the whipstock out of proper angular orientation. This drawback has seriously limited the use of downhole motors for such applications. Therefore, a need exists for a downhole motor assembly that may be temporarily “locked” as the bottomhole assembly is run into the well bore, oriented and set into position, and subsequently selectively “unlocked” for operation.
Further, although a low flow rate of fluid is sufficient for hydraulically setting an anchor or packer below the motor, the surface pumps may be designed to displace fluid at the drilling flow rate absent any flow restriction. Fluid displacement at the drilling flow rate will tend to drive an unlocked motor, and may be sufficient to unlock a temporarily locked motor. Therefore, a need exists for a downhole motor assembly that temporarily restricts the flow rate of fluid through the locked motor during the setting procedure.